feat(showcase): PII section — mask + elfproef parse (WP-42, mask/parse half)

Add a "PII — maskeren & parsen" section to /concepts demonstrating the WP-40 pieces
with FP + atomic design, framed for AVG art. 9 / data-minimisation: a live
<app-masked-value> atom (masked BSN that reveals on click; real reveal is step-up +
audited in behandel-scherm) and a live parseBsn elfproef parse. Both show the real
linked source via the WP-39 snippet mechanism (new showcase regions in bsn.ts + pii.ts,
registered in gen-snippets.mjs). Delivers WP-42's showcase demo; the persisted-audit
half stays pending WP-41.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/shared/kernel/bsn.ts

@@ -11,6 +11,7 @@ export type Bsn = Brand<string, 'Bsn'>;
 // Positional weights for the elfproef: 9·d1 + 8·d2 + … + 2·d8 − 1·d9 ≡ 0 (mod 11).
 const WEIGHTS = [9, 8, 7, 6, 5, 4, 3, 2, -1];
 
+// #region showcase:parseBsn
 export function parseBsn(raw: string): Result<string, Bsn> {
   const t = raw.trim();
   if (!/^\d{9}$/.test(t)) {
@@ -22,5 +23,6 @@ export function parseBsn(raw: string): Result<string, Bsn> {
       $localize`:@@validation.bsnElfproef:Dit is geen geldig BSN (klopt niet met de elfproef).`,
     );
   }
-  return ok(t as Bsn);
+  return ok(t as Bsn); // holding a Bsn is proof it passed the elfproef
 }
+// #endregion showcase:parseBsn

src/app/shared/kernel/pii.ts

@@ -5,6 +5,7 @@
  */
 export const REDACTED = '‹redacted›';
 
+// #region showcase:mask
 /** Keep the last `keep` characters, mask the rest with `*`. */
 export function maskTail(value: string, keep: number): string {
   if (value.length <= keep) return '*'.repeat(value.length);
@@ -15,3 +16,4 @@ export function maskTail(value: string, keep: number): string {
 export function maskBsn(value: string): string {
   return maskTail(value, 3);
 }
+// #endregion showcase:mask